SECTION III, 1883. [ 49 ] Trans. Roy. Soc. CANADA. 
On some Experiments shewing that the Electromotive Force of Polarization is independent 
of the difference of Potential of the Electrodes, By Professor J. G. MAcGREGOR, 
MA, DSc., F.R.S.E. 
(Read May 22nd, 1883.) 
At the last meeting of the Royal Society of Canada, I read a paper before this section 
on a new method of measuring the resistance of electrolytes. I pointed out at the time 
that this method involved the assumption that the electrolytic polarization of electrodes 
was independent of their difference of potential during the passage of the polarizing 
eurrent—at any rate, in the special case in which the polarizing current flowed for short 
periods of time. Various experimenters had shewn it to depend upon the density of the 
current, e., upon the strength of the current per unit area of electrode. But, so far as I 
knew, no investigations had been made to determine whether or not it varies with the 
difference of potential of the electrodes. To settle this question, I have made a series of 
experiments of which this paper contains an account. 
For this purpose, I compared the strengths of the polarization produced by currents of 
the same density, flowing during the same time through two electrolytic cells containing 
the same electrolyte and electrodes of the same substance, but at different distances. To 
get currents of the same density, I used electrodes of exactly the same area and polarized 
them in all cases by joining them up in series as parts of one circuit and passing therefore 
the same current through both. That the difference of potential of the electrodes of the 
respective cells during polarization might be different, it was only necessary that the 
resistance of the electrolyte between the electrodes should be different, 7.e., as the cells 
had the same section, that the distances between the electrodes of the cells should be 
different. In the cells used, therefore, the distances of the electrodes were made capable of 
variation. 
Electromotive force may be measured in three ways. Wheatstone’s method, which 
involves the use of a galvanometer and applies Ohm’s Law, or a modification of it, has 
frequently been employed. It has this advantage that the polarization is measured during 
the flow of the current. But it can be used for the purpose of determining the electromotive 
force of polarization only in the case in which the polarizing current has been flowing so 
long that the electromotive force of the polarization has reached its maximum value. This 
method therefore was unsuitable for my experiments. Poggendorff’s compensation method 
and the method of direct electrometric measurement are the other two. Both are charac- 
terized by the distinct disadvantage that the measurement of the electromotive force of 
polarization is made after the cessation of the polarizing current and that the value deter- 
mined is less than its value at the moment at which the polarizing current ceased, which 
is the value desired. This disadvantage is most formidable when the polarized cells under 
investigation are polarized up to or nearly up to the maximum. When this is so, the rate 
of the dissipation of the polarization after the polarizing current has ceased to flow is very 
Sec. III., 1883. 7. 
